1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, a printer, a plotter, a facsimile machine, or a multifunction device having two or more of copying, printing, plotting, and facsimile functions, and more particularly to a full-color image forming apparatus having both color and monochrome image forming units and a configuration capable of driving just the monochrome image forming unit provided therein to form a monochrome image.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, printers, plotters, facsimile machines, and multifunction devices having two or more of copying, printing, plotting, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
Full-color image forming apparatuses that form both monochrome and full-color images widely employ a tandem system. In the tandem system, multiple image forming parts each forming a toner image of a specific color are arranged side by side relative to a transfer medium. The toner images of the specific color respectively formed by the multiple image forming parts are sequentially transferred onto the transfer medium to form a full-color toner image on the transfer medium.
The tandem system may use either a direct transfer system, in which the toner images are directly transferred onto the last transfer medium such as a sheet of paper to form a full-color image on the sheet, or an indirect transfer system, in which the toner images are primarily transferred onto an intermediate transfer body to form a full-color toner image on the intermediate transfer body, and then the full-color toner image is secondarily transferred onto the sheet, that is, the last transfer medium, from the intermediate transfer body to form a full-color image on the sheet. The indirect transfer system can handle various types of recording media and reliably provide higher-quality images compared to the direct transfer system. Accordingly, the indirect transfer system has become more common in recent years.
However, in the above-described full-color image forming apparatuses, the intermediate transfer body, a cleaning member that cleans the intermediate transfer body, and so forth, must be operated in a monochrome mode to form a monochrome image in the same manner as in a full-color mode. Further, even in the monochrome mode, a period of time equal to that taken in the full-color mode for forming a full-color image is required. There are structural reasons for these outcomes, as described below.
The related-art image forming apparatuses include a first image forming unit that forms only a monochrome toner image on a recording medium and a second image forming unit that forms multi-colored toner image on a recording medium, in order to be able to handle formation of both monochrome and full-color images. In the monochrome mode, the second image forming unit is retracted so that only the first image forming unit is operated to form a monochrome image. Alternatively, the first image forming unit may be separately provided from the second image forming unit. However, movement of the second image forming unit degrades positional accuracy of rollers used in the full-color mode to form a full-color image.
In another approach, a conveyance path of the recording medium for the monochrome mode is separated from that for the full-color mode. However, this approach, although effective for the intended purpose, results in a complex and therefore costly configuration.
In yet another approach, a conveyance belt that conveys a recording medium having a monochrome toner image thereon to an intermediate transfer belt on which a multi-colored toner image is formed is movably provided to contact to or separate from the intermediate transfer belt. However, such movement of the conveyance belt may change an angle of entrance of the recording medium into the fixing device to which the recording medium is ultimately conveyed by the conveyance belt, causing image deterioration and possibly wrinkling the recording medium. In order to reduce mechanical movement of the conveyance belt, a gap formed between the conveyance belt and the intermediate transfer belt upon separation of the conveyance belt from the intermediate transfer belt is required to be smaller. However, the smaller gap between the conveyance belt and the intermediate transfer belt may cause the monochrome toner image formed on the recording medium to adversely contact a surface of the intermediate transfer belt while the recording medium is passing through the gap, or may cause toner of the monochrome toner image formed on the recording medium to scatter toward the intermediate transfer belt even when the monochrome toner image does not contact the intermediate transfer belt.